This invention relates to a method for detecting a defect in a glass-ceramic for magnetic disks used as recording media in computers.
There has been a rapid progress in developments in magnetic disk for which demand has recently increased remarkably as external media of large scale computers and personal computers.
Aluminum alloy has been conventionally used as a material of magnetic disk substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness and surface roughness and cannot cope with the recent requirement for high density recording necessitated by increase in the amount of information.
Since a glass-ceramic can be formed to a fine structure containing constituent crystal grains having diameters ranging from 0.02 to 20 .mu.m, it is superior to aluminum alloys in flatness and surface roughness and therefore is more suited to the high density recording. For this reason, magnetic disk substrates made of a glass-ceramic have been proposed and offered on the market.
In a case where a magnetic disk substrate is formed with a glass-ceramic, defects such as a flaw, crack, break and pin hole, if existing in the glass-ceramic, will adversely affect the properties of the magnetic disk produced, sometimes making the magnetic disk unusable depending upon the magnitude of the defects. In manufacturing the prior art glass-ceramics for magnetic disk substrates, therefore, the surface of the glass-ceramics produced is visually checked for finding out such defects.
It is, however, difficult to find out such defects by a visual checking in the prior art glass-ceramics which are transparent or opaque and this visual checking imposes a heavy burden on a person who visually checks the glass-ceramic products and sometimes causes an error in checking. There is an automatic checking method available for checking glass products by using laser beam but, in the case of the glass-ceramics, the difficulty in finding out defects in the glass-ceramics remains unsolved even by using the automatic checking method. Therefore, this difficulty in checking defects in glass-ceramic products poses a serious problem in the quality management of glass-ceramic products.
It is, therefore, an object of the invention to provide a method for detecting a defect in a glass-ceramic for magnetic disks which enables defects on the surface of the glass-ceramic to be detected easily.